XHR isn't a content type, it is how the request is initiated. The content requested was HTML which is why that was the content type.

As far as not blocking further requests, the dev tools screen shots appear to show the exact same thing. Requests after MCA21/ only start after it finished. Presumably because the code that actually triggered the XHR is doing the work that causes the other requests to be created.

Same goes for the tmp.html XHR request. There are quite a few requests in the dev tools waterfall that look like they are not loaded until that XHR completes.